The present invention relates to a process for the preparation of 1,4,7,10- tetraazacyclododecane of formula (I) starting from decahydro-2a,4a,6a,8a- tetraazacyclopent fgacenaphthylene of formula (II): (I)comprising a direct step of hydrolysis in aqueous solution, at temperatures ranging from 60 to 150°C., in conditions of slightly acid, neutral or slightly basic pH, in the presence of 2-20 mol of diamines of formula (V) per mole of (II), wherein: x ranges between 0 and 2, and Q is -CH2CH(OH)CH2-, or-(CH2)2NH(CH2)2-, OR-(CH2)2NH2(CH2)2, when x is 1, or Q is -CH2- when xis 1,or Q is-CH2-When X is 2.